In face of increasingly fierce competition in the market, the efficiency of the base station products has become the focus of competition in the industry, the efficiency of the main component—power amplifier—that determines the efficiency of the base station has become a top priority, and the industry has invested in the research on the efficiency improvement technologies, wherein, the Doherty technology is a mature technology that is most widely used at present, and the amplifier manufacturers have begun producing and applying the Doherty amplifiers in mass, how to further improve the efficiency in this technology is particularly important.
The Doherty technology was invented by W. H. Doherty in 1936, it was originally used in traveling wave transistors to provide high power transmitter for broadcasting, its structure is simple and highly efficient.
The conventional Doherty structure consists of two power amplifiers: a main power amplifier (also called the carrier power amplifier) and an auxiliary amplifier (also known as Peak Power Amplifier), wherein, the carrier power amplifier works in Class B or AB, and the peak power amplifier works in Class C. The two power amplifiers do not work in turns, but the main amplifier works all the time, and the auxiliary amplifier only works when the preset peak power is reached. The 90 degree quarter-wavelength line after the carrier power amplifier is for impedance transformation, its purpose is to play the role of reducing the apparent impedance of the carrier power amplifier when the auxiliary power amplifier works, thus to ensure that the active load impedance consisting of the auxiliary power amplifier and the subsequent circuits reduces when the auxiliary power amplifier works, thus the output current of the carrier power amplifier is amplified. Due to the quarter-wavelength line after the main power amplifier, in order to make the outputs of the two power amplifiers in phase, 90° phase shift is also needed before the auxiliary power amplifier, as shown in FIG. 1.
The main power amplifier works in Class B, when the input signal is relatively small, only the main power amplifier is in a working condition; when the output voltage of the main power amplifier transistor reaches the peak saturation point, the efficiency can reach 78.5% in theory. If the excitation is doubled at this time, the main power amplifier is already saturated when a half of the peak value is reached, and the efficiency also reaches 78.5% of the maximum, at this time, the auxiliary power amplifier also begins to work together with the main power amplifier. The introduction of the auxiliary power amplifier makes the load reduced from the perspective of the main power amplifier, since the auxiliary power amplifier for the load is equivalent to connecting a negative impedance serially, even if the output voltage of the main power amplifier is saturated and constant, the output power continues to increase (the current flowing through the load becomes larger) due to the load reduction. When the excitation peak is reached, the auxiliary power amplifier also reaches the maximum point of its own efficiency, and the total efficiency of the two power amplifiers is much higher than the efficiency of a single class B power amplifier. The maximum efficiency 78.5% of a single class B power amplifier appears at the peak value, but currently the efficiency 78.5% appears at a half of the peak value, so this kind of system architecture can achieve very high efficiency (each amplifier reaches its maximum output efficiency).
Due to the requirements of the base station system on set-top output power, the requirement for the gain of RF power amplifier is tens of dB, thus one stage of amplification is not enough, generally 3-4 stages of amplification are needed, that is, the pre-drive, the drive and the final stage. Currently, the link structure commonly used in the industry is as follows: the pre-drive stage uses the RF small-signal amplifier, and its working mode is CLASS A; the drive and final stages use the same type of RF power amplifier transistors (currently, the industry uses the LDMOS devices), the working mode of the drive stage is CLASS AB, and the final stage is the Doherty structure.
With the industry's green concept, the requirements by the operators on the efficiency of the communication system is almost harsh, even with the advanced Doherty technology, the power amplifier efficiency is still unable to meet their increasing demands, it is necessary to make continuous improvements on the basis of the Doherty technology to achieve continuous efficiency improvement.
In the traditional RF power amplifier, the Doherty structure is only applied to the final stage, and the drive stage and the final stage use the same type of power amplifier transistors, whose advantages are: the supply voltages and the bias modes are the same, thus the design of the bias circuit is simple; since the amplifier transistors are the same type, the discretion of the mass production is relatively easy to control. However, a fact that cannot be ignored is: the industry's leading LDMOS device has been developed to the eighth generation, its cost is low, but its performance has very limited room for improvement, which cannot meet the environmental protection requirements; in addition, although the power amplifier efficiency is primarily determined by the final stage, the final stage contributes 90% of the operating current, thus further enhancing the efficiency of the final stage has great significance, but the 10% contributed by the drive stage cannot be ignored even more, therefore, it also needs to improve the circuit of the drive stage.
From the signal power spectrum distribution of different schemes of the current communication systems, the 70%-80% energy output by the power amplifier is concentrated around the average power, that is, most of the operating current of the final stage power amplifier which applies the Doherty technology is contributed by the Carrier amplifier, thus enhancing the efficiency of the final stage Carrier amplifier has great significance in improving the efficiency of the entire power amplifier. Meanwhile, the efficiency can be further improved in the drive stage part, which can also better realize the efficiency improvement of the entire power amplifier.